compositng: when I use an alpha over node only one layer shows up
Your image renders properly when you select "Transparent' within the Cycles 'Film' section.... In the Blender file available for download, it is unchecked...

When checked, this is the result I get... and nice sparks!



Sorry... was not sure if it was in or out....  Is this as you intended? See Gears6 blend file... if this is correct, the issue was in the node tree.... but, if not, I misunderstood. :)